A power amplifier used in a transmitter of a mobile terminal is designed and sold so as to obtain optimal performance when antenna load impedance is specific impedance (e.g., 50 ohms). Referring to FIG. 1 showing a circuit of a general power amplifier, assuming that antenna load impedance RL is 50 ohms, a power amplification circuit and an output matching circuit are designed. Impedance ZLOAD toward an antenna load from an output terminal of the power amplification circuit (hereinafter, referred to as ‘amplifier load impedance’ to be distinguished from the antenna load impedance) is a major parameter for determining the performance of the power amplifier, and has important influence on output power, efficiency, linearity and the like of the power amplifier.
However, when used in practice, the antenna load impedance of the power amplifier does not become 50 ohms and is extremely variable depending on a user and ambient environment. The change in the antenna load impedance results in change in the amplifier load impedance ZLOAD, and therefore, many problems occurs such as deterioration of the performance of the power amplifier, an increase in the power consumption of the mobile terminal and a reduction in battery lifespan.